研究業績

During the Paleogene, Earth experienced a significant transition from a hot to a cold climate, or from a “Hothouse” to a “Coolhouse.” In the warm early Paleogene, the oceanic environment lacked a large polar ice sheet and had a reduced equator-to-pole sea-surface temperature gradient. Large-scale tectonic events occurred in the high-latitude South Pacific during this period, such as the northward movement of Zealandia away from the Antarctic continent, the deepening of the Tasman Sea accompanied by seafloor spreading, and the opening of the Tasman Gateway. However, variations in oceanic circulation and depositional environments in the high-latitude South Pacific associated with global climate change or tectonic settings during the Paleogene have not been fully characterized. Here, we report the chemical compositions and Sr-Nd isotopic ratios of carbonate fractions and bulk sediments from International Ocean Discovery Program Site U1553. The rare earth element patterns and Sr isotopes of the samples suggest that, since 62 Ma, Site U1553 was located in the open ocean rather than on continental shelves or margins. Leachate Nd isotopic data indicate that intermediate water from the South Indian Ocean flowed north of Australia and onto the Campbell Plateau throughout the Eocene, probably via the proto-Eastern Australian Current (Eastern Australian Current) since 52 Ma. This circulation pattern was likely linked to the opening and deepening of the Tasman Sea. Seawater around the Campbell Plateau had the low εNd values during the Eocene, which may have contributed to the development of an εNd gradient between the South and North Pacific Oceans.